There are theoretical and practical problems to be solved concerning the characterization of hearing sensitivity and sound discrimination abilities in infants. Knowledge of these hearing capabilities is key to determining appropriate habilitation strategies when infants are detected with hearing loss. This project addresses the challenge of characterizing hearing in infants using electrophysiologic responses to amplitude and frequency modulated tones. In conjunction with electrophysiologic methods, age-specific psychophysical measures will be employed to estimate threshold and discrimination abilities in young infants. If we are successful, it may be possible to relate electrophysiologic correlates of hearing sensitivity and central auditory nervous system maturation to perceptual abilities. The long-term goals of this research are to understand how and when infants develop the capacity to detect, discriminate and perceive speech and the evaluation of this development through electrophysiologic and psychophysical methods. The building blocks of speech understanding are defined as 1) hearing sensitivity for "speech frequencies" (500-4000 Hz), 2) temporal processing as reflected in neural phase-locking for modulated tones in the speech frequencies; 3) sensitivity to and discrimination of amplitude modulation and frequency modulation; 4) discrimination of complex sounds composed of two or more modulated components. The research plan is developed to explore these building blocks of speech understanding using auditory electrophysiology and psychophysics to describe the time course and dynamics of responsiveness to amplitude (AM) and frequency (FM) modulated tones and complex sounds composed of multiple carder and modulation frequencies. The research outcomes will contribute new knowledge about how the human auditory system develops the capacity to respond to and discriminate complex signals. This research will also result in methods for assessing threshold sensitivity and sound discrimination in infants that can be applied to those at risk for hearing loss or other auditory dysfunction.